The pre-T Cell Receptor (pre-TCR) is a receptor expressed early during the development of T cells in the thymus. It's signaling is essential for the maturation of T cells as it induces proliferation and differentiation through downstream signaling pathways that involve the activation of key transcription factors. One of the main functions of the pre-TCR is the induction of cell survival as cells unable to express the receptor die by programmed cell death. However, the mechanisms of both apoptotic death of pre-TCR non-expressing thymocytes and the pre-TCR mediated survival are poorly understood. In this proposal we attempt to identify the molecular mechanisms responsible for the induction of survival and study the role of well-characterized death inducers in the apoptotic cascades of pre-T cells. To achieve that we a) analyze biochemically the constituents of the apoptotic pathway in these cells, b) study thymic phenotypes of mice deficient for several genes - key components of the apoptotic machinery, and c) characterize the nature of pre-TCR derived antiapoptotic signals and in particular the expression pattern and the function of the pre-TCR-induced prosurvival factor A1 and the pre-TCR-suppressed pro-apoptotic Bim. We are also linking constitutive pre-TCR signaling, that leads to deregulated survival and proliferation, to the induction of T cell acute lymphoblastic leukemia (T-ALL). To study the role of the pre-TCR in thymocytes transformation we will a) address the nature of interactions of oncogenes (Notch) with pre-TCR signaling, b) study the significance of pre-TCR gene-targets in the triggering of transformation and c) attempt to identify additional genetic events contributing in the establisment of the disease. The proposed experiments will help to identify the molecular mechanisms regulating pre-T cell survival and death and pinpoint pre-TCR-induced signaling pathways responsible for the malignant transformation of thymocytes.